Polyolefin coated steel pipes with a polyolefin coating consisting of linear low density polyethylene (JP 08,300,561), blends of propylene polymers and α-olefin copolymer elastomers (JP 2000,44,909) or syndiotactic polypropylene (JP 08,300,562) are known. The disadvantage of these polyolefin steel coatings is the insufficient dynamic fracture toughness of test pipes fabricated from the coating material. A high dynamic fracture toughness is required for coated steel pipes in order to avoid cracking of the coating during installation handling and in service.
The term installation handling as used herein means any installation technique such as coiling and uncoiling of the ready made pipelines, welding and other jointing techniques and installation at the seabottom for off-shore intallations with specially designed ships, most often to a depth of several hundreds of meters, also to uncertain sea bottom conditions with risk of rock impingements etc. Installation handling of coated steel pipes, in particular for off-shore applications, involves tough conditions for the protective coating layer, including high stress, substantial elongation, surface damages, notches, impact events etc, both at low and high temperature conditions and also at high hydrostatic pressure. The coating layer is not only the layer protecting the pipeline as such from damages as mentioned, it is also doing so in a stage of high stress and/or at elevated temperatures and pressures, making the coating layer most sensitive for cracking, compare in particular the stresses induced during coiling and uncoiling. During the service life of the coated pipeline, the coating has to protect the pipeline from damages and induced stress and crack formations at conditions close to 0° C., high hydrostatic pressures where a small damage or notch in the coating could propagate into a large crack putting the pipeline as such at risk. With a high dynamic fracture toughness of the coating material the material will not crack during installation handling and in service.